Running Away from Cancer

Does exercise help protect against cancer?

There are plenty of theories about how exercise might reduce the risk—focusing, for instance, on its ability to control weight, lower hormones and cellular growth factors, improve insulin resistance, and, when done in moderation, enhance the immune system.

Over the years many (though not all) studies have found a link between physical activity and reduced cancer risk. But many questions remain.

The best evidence of potential benefit concerns colon cancer. For instance, a 2009 analysis in the British Journal of Cancer, which combined the results of 52 studies, found that physical activity reduces the risk by 24 percent, on average. Even here, though, we still don’t know how much exercise is needed or whom it might help most.

The effect on breast cancer risk has also been studied exten­sively, with mostly positive results. A 2009 review of research on exercise and breast cancer prevention concluded that the evidence is “compelling.”

For prostate, lung, and endometrial cancers, research has been promising, though less consistent. Exercise has also been found to improve the prognosis and well-being of people already diagnosed with cancer.

Why can’t we figure this out?

Cancer is many diseases, has many contributing factors, and gener­ally takes many years to develop. To prove that physical activity pro­tects against breast cancer, for example, scientists would have to put hundreds of women on various exercise regimens (including no exercise) and see what happens over years and even decades. Obvi­ously, that is impossible.

Instead, researchers largely rely on popula­tion studies that ask people to describe their long-term exercise habits. But it’s hard to remember accurately what you did last week, let alone 10, 20, or 30 years ago.

Moreover, the studies have included all different kinds of physi­cal activity, making it hard to compare their results. Some have looked only at recreational exercise, but have not differentiated between high-intensity and less strenuous activities.

Others have included occupational and/or household activities. And since peo­ple who exercise tend to do lots of other healthy things—have a good diet, for instance—it’s hard to tease apart the effects of all these factors.

It’s still not clear whether people need to start young and exer­cise their whole lives to reduce their cancer risk, or if becoming active later in life is beneficial, too. Genetics undoubtedly helps determine who benefits most.

Bottom line:It’s unlikely that exercise alone can prevent cancer, but it does seem to improve the odds. The American Cancer Society, by the way, recommends at least 30 minutes of moderate-intensity activity five or more days of the week. That’s good advice.